Alternative energy systems can be classified according to whether they are stand-alone systems or grid-connected systems. Mostly, a stand-alone system is used in off-grid applications with battery storage. In a grid-connected system, excess power can be sold to an electric utility or “the Grid”, typically in the afternoon hours of the day which happen to coincide with peak rate times. When the grid-connected system is generating less than the consumed amount of power, the Grid continues to supplement the power generated by the alternative energy system.
Grid-interactive inverters (commonly referred to as grid-tie inverters) are a type of power inverter that converts direct current (DC) into alternating current (AC) that is fed to the Grid. Current will flow from the inverter to the Grid when the instantaneous voltage supplied at the inverter outputs exceeds the instantaneous grid voltage.
When a photovoltaic module is the source of the direct current, a grid-interactive inverter inverts a relatively low and variable DC voltage to a relatively high AC voltage that is matched to the Grid. A variety of grid-interactive inverter structures can be utilized in applications involving photovoltaic modules including grid-interactive inverters that are derivatives of a basic H-bridge topology and structures that are derivatives of a neutral point clamped (NPC) topology. A typical grid-interactive inverter includes a stage that converts DC voltage to AC voltage using switches that switch current in a bidirectional manner across the output terminals of the grid-interactive inverter to provide AC to the Grid. The switches are typically implemented using transistors, which are controlled using pulse width modulation (PWM) signals that define the periods of time in which individual transistors are ON or OFF. When the switches are controlled in an appropriate manner and the voltage drop across the output filter is sufficiently large relative to the Grid voltage, the bidirectional flow of current through the output filter results in a sinusoidal current at the output of the grid-interactive inverter that is compatible with the Grid.
In many implementations, the DC voltage received by the grid-interactive inverter does not exceed the peak voltage of the Grid and so a direct DC-AC inversion is not performed. Instead, multiple stages are utilized within the grid-interactive inverter that boost the received DC voltage to a DC voltage exceeding the rectified voltage of the Grid, and invert the boosted DC voltage to provide AC to the Grid. A common technique for boosting the DC voltage received from a photovoltaic module is to convert the DC to AC and to utilize an appropriately wound transformer to step the AC voltage up to a higher voltage. The stepped up AC output can be full wave rectified to provide a DC voltage to the DC-AC inverter stage that exceeds the peak voltage of the Grid. In many implementations, the DC-DC conversion stage utilize switches that switch current in a bidirectional manner through the primary coil of a transformer. The output of the secondary coil can then be full wave rectified to accumulate charge on a DC link capacitor. The DC link capacitor serves as an energy buffer. The peak current draw on the DC link capacitor by the DC-AC inverter stage typically exceeds the current provided to the DC link capacitor by the DC-DC conversion stage. Therefore, the DC link capacitor stores enough charge to meet the peak current draw of the DC-AC inverter stage and enable power to be exported by the grid-interactive inverter throughout each grid cycle. The switching of current through the primary coil of the transformer by transistors in the manner outlined above can be controlled using PWM control signals. As can readily be appreciated, the AC in a DC-DC conversion stage need not have a frequency and/or phase matched to the Grid. Instead, the frequency and/or phase of the AC can be determined based upon the performance of the DC-DC conversion stage.
The PWM control signals that drive the switches in the various stages of a grid-interactive inverter are typically generated by a controller that monitors the Grid voltage and adjusts the switching of the DC-AC inverter stage to produce a current compatible with the Grid. The presence of a controller within a grid-interactive inverter can enable other functionality targeted at improving the efficiency and/or power output of the inverter. For example, photovoltaic modules typically have a non-linear output efficiency that can be represented as an I-V curve. The I-V curve provides information concerning the current that the inverter should draw from the photovoltaic module to obtain maximum power. Maximum power point tracking is a technique involving application of a resistive load to control the output current of a photovoltaic module and maximize power production.
Micro-inverters are a class of grid-interactive inverter that converts a DC voltage from a single photovoltaic module to an AC voltage. A key feature of a micro-inverter is not its small size or power rating, but its ability to perform maximum power point tracking to control on a single panel. Micro-inverters are commonly used where array sizes are small and maximizing performance from every panel is a concern.
Where panels are connected in series, a string inverter can be utilized. A benefit of connecting panels in series in this way is that the DC voltage provided to the string inverter can be sufficiently high so as to exceed the peak grid voltage. As noted above, a single stage inverter can be utilized when the DC input voltage exceeds the peak grid voltage. Typically, a single stage inverter is more efficient than a multiple stage inverter due to energy losses that occur in DC-DC conversion stages associated with the transformer and switching losses. String inverters are typically used with larger arrays of photovoltaic modules.